The present invention is related to swimming flippers comprising a flexible but relatively rigid blade and a relatively soft shoe associated to one end of the blade, and wherein the blade has an active portion formed by two materials.
Examples of swimming flippers of this type are generally known, for instance, from U.S. Pat. No. 4,820,218 and U.S. Pat. No. 5,163,859, wherein the two materials have a stratified configuration.
More particularly, the invention is directed to a swimming flipper of the above-mentioned type in which a first of the said two materials is a thermoplastic material and a second material occupies areas wherein said active portion of the blade is devoid of the first material.
Flippers of the above-referenced type are known, for instance, from U.S. Pat. No. 3,411,165, U.S. Pat. No. 4,738,645, U.S. Pat. No. 4,887,985 and U.S. Pat. No. 4,954,112.
According to U.S. Pat. No. 3,411,165, U.S. Pat. No. 4,738,645 and U.S. Pat. No. 4,887,985 the first material defines the relatively rigid load bearing structure of the blade, and the second material is a relatively thin and supple rubberlike material, defining webs having a non-stretched and baggy configuration. The webs are longitudinally secured to the first material by means of flexible joinders and define tapered, cup shaped pockets extending along the substantially rigid structure defined by the first material, which pockets tend to transversely bow or belly both downwardly and upwardly. The transverse bowing of the webs progressively increases as they extend forward from the shoe.
According to U.S. Pat. No. 4,738,645 and U.S. Pat. No. 4,887,985 the areas of the active portion of the blade devoid of the first material are constituted by two narrow slits oriented along the longitudinal direction of the blade and placed at a short distance from the lateral edges thereof, or by at least one narrow central slit, respectively.
The two slits, or the central slit, are occupied by respective highly flexible, extensible and supple membranes, constituted by the same thermoplastic rubber material of the shoe. The membranes act like hinges so as to allow relative displacement between the central area of the active portion of the blade and the two lateral areas, and when using the flipper make the central portion of the blade dynamically deformable in opposition with the swimming stroke, so as to generate thereby a liquid flow conveying canal the concavity of which is inverted as soon as the swimming stroke is reversed.
In the case of U.S. Pat. No. 4,954,112, two longitudinal slits are also provided along the active portion of the blade, which are occupied by two rubber inserts the function of which is, according to the inventor, that of providing warping of the blade so as to enable the latter, in use, to be constantly kept parallel to the displacement direction of the swimmer, thus preventing gliding of the flipper transversely.
These known flippers have several drawbacks.
Firstly, the great elastic deformability and extensibility of the rubber elements (membranes or inserts) placed along the longitudinal slits of the blade, while allowing on one hand achieving a positive effect of longitudinal calalizing the water flow during flipper motion, on the other hand involve an excessive "hinge" deformability between the different areas of the active portion of the blade separated from one another by the slits, which negatively affects the dynamic thrust power of the blade. In practice, the central portion of the blade is always displaced with a certain delay with respect to the periphery thereof, which limits in use sudden accelerations or "sprinting". The flipper is not fatiguing, but supple and of low performance.
Moreover, the rubber or thermoplastic rubber which is employed for the membranes or inserts has several defects: besides high cost both of the material itself and of the technology which are necessary for application thereof to the thermoplastic material of the remaining portion of the blade, it has a poor aging resistance, which can be improved only by means of additives giving thereto a dark grey or black colouring. As a consequence, the possibility of obtaining on the blade chromatic effects, not only pleasant from the aesthetical point of view but even such as to improve safety in use by means of colourings adapted to facilitate sighting of a diver wearing the flippers also in unclear water or dim light, are scarce.
Further the rubber is characterized by a low coefficient of elasticity. The reduced cross-section of the membranes and of the inserts according to the above prior art contributes in a very limited way to the global moment of inertia of the blade cross-sections and, therefore, in order to manufacture flippers having a sufficient rigidity it would be necessary to increase the thickness of such membranes and inserts, with a consequent burden in terms of weight and costs.
A further inconvenience, which is common to nearly all the flippers presently known, resides in the high cost for moulding the blades, deriving from the presence therein of longitudinal stiffening ribs and/or ridges, whose thickness (normally in the range of 8-10 mm) is appreciably greater than the general thickness of the blade (normally in the order of 2-4 mm). Actually, when drawing the blade out of the mould the core material within the said ribs and/or ridges is still fluid or anyway too hot, thus obliging, in order to avoid possible material shrinkage cavities and consequent negative effects both on the dynamic behaviour when using the flipper and from the aesthetic point of view, to increase the moulding cycle time so as to ensure the necessary cooling.